In electrostatography an image comprising an electrostatic field pattern, usually of non-uniform strength, (also referred to as an electrostatic latent image) is formed on an insulative surface of an electrostatographic element by any of various methods. For example, the electrostatic latent image may be formed electrophotographically (i.e., by image wise photo-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on a surface of an electrophotographic element comprising a photoconductive layer and an electrically conductive substrate), or it may be formed by dielectric recording (i.e., by direct electrical formation of an electrostatic field pattern on a surface of a dielectric material). Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrostatographic developer. If desired, the latent image can be transferred to another surface before development.
One well-known type of electrostatographic developer comprises a dry mixture of toner particles and carrier particles. Developers of this type are commonly employed in well-known electrostatographic development processes such as cascade development and magnetic brush development. The particles in such developers are formulated such that the toner particles and carrier particles occupy different positions in the triboelectric continuum, so that when they contact each other during mixing to form the developer, they become triboelectrically charged, with the toner particles acquiring a charge of one polarity and the carrier particles acquiring a charge of the opposite polarity. These opposite charges attract each other such that the toner particles cling to the surfaces of the carrier particles. When the developer is brought into contact with the latent electrostatic image, the electrostatic forces of the latent image (sometimes in combination with an additional applied field) attract the toner particles, and the toner particles are pulled away from the carrier particles and become electrostatically attached imagewise to the latent image-bearing surface. The resultant toner image can then be fixed in place on the surface by application of heat or other known methods (depending upon the nature of the surface and of the toner image) or can be transferred to another surface, to which it then can be similarly fixed.
Many well-known types of toner useful in dry developers comprise binder polymer materials such as vinyl addition polymers or condensation polymers. Such binder polymers are chosen for their good combinations of advantageous properties, such as toughness, transparency, good adhesion to substrates, and fusing characteristics, such as the ability to be fixed to paper at relatively low fusing temperatures while not permanently adhering to fusing rolls, except at relatively high temperatures. As is well-known, vinyl addition polymers that are useful as binder polymers in toner particles can be linear, branched or lightly crosslinked. The most widely used condensation polymers are polyesters which are polymers in which backbone recurring units are connected by ester linkages. Like the vinyl addition polymers, polyesters useful as binder materials in toner particles can be linear, branched, or lightly crosslinked. They can be fashioned from any of many different monomers, typically by polycondensation of monomers containing two or more carboxylic acid groups (or derivatives thereof, such as anhydride or ester groups) with monomers containing two or more hydroxy groups.
While many binder polymers exhibit many desirable properties for use in electrostatographic toners, they do have certain shortcomings. For example, binder polymers are commonly ground to a small particle size to provide the high degree of resolution required in good quality copies. Accordingly, such polymers are desirably very brittle which provides a high degree of grindability. Unfortunately, many polymers, and especially polyesters which are otherwise useful for toners are not sufficiently easily ground to the very small particle sizes needed for high-resolution toners. To overcome this problem, methods have been developed which directly provide binder polymers having a controlled and predetermined size and size distribution suitable for use in electrostatographic toners. One such method is a polymer suspension technique which is known in the prior art as a "limited coalescence" process, as described in U.S. Pat. No. 4,833,060, issued May 23, 1989 and U.S. Pat. No. 4,965,131, issued Oct. 23, 1990.
In the limited coalescence process described in the aforementioned patents, binder polymer particles are prepared by forming a solution of a suitable polymer in a solvent that is immiscible with water, dispersing the polymer solvent solution in an aqueous medium containing small particles of a solid stabilizer to form droplets of polymer solvent solution (oil phase) in the aqueous medium (aqueous phase), removing the solvent and recovering binder polymer particles. During the course of the process, coalescence of the oil (discontinuous) phase takes place to form larger size droplets. These droplets are limited in size by the presence of the particles of the solid stabilizer (often referred to as a suspension stabilizing agent) in the aqueous (continuous) phase. The solid stabilizer particles limit coalescence from taking place by a mechanism that is generally believed to be a physical phenomenon of preventing, by separation, one droplet from wetting another and thereby joining together to form a larger droplet. Removal of the solvent from the droplets provides solid binder polymer particles that are covered with a layer of smaller stabilizer particles.
In the limited coalescence process described in U.S. Pat. No. 4,833,060 the solid stabilizer particles are silica particles which are removed from the binder polymer particles that are used in an electrostatographic toner. This is necessary because the presence of stabilizer particles such as silica interfere with the triboelectric relationship between carrier particles and toner particles employed in developers in electrostatographic copying devices.
U.S. Pat. No. 4,965,131 represents a significant improvement in making binder polymer particles by limited coalesence-type processes in that polymeric stabilizer particles are employed by the patentees and such stabilizer particles need not be removed from the surfaces of the binder polymer particles. Accordingly, this process avoids the lengthy and generally costly techniques that are normally necessary to remove stabilizer particles such as silica from binder polymer particles intended for use in electrostatographic toners. Other significant advantages of substituting polymeric stabilizer particles for inorganic stabilizer particles such as silica in a limited coalescence process are described in U.S. Pat. No. 4,965,131.
It is also known that electrostatographic toners used in dry developers often contain material referred to as a charge agent or charge-control agent, which helps to establish and maintain toner charge within an acceptable range. Many types of charge-control agents have been used and are described in the published patent literature. Unfortunately, many of the known charge-control agents exhibit significant drawbacks when used with binder polymer particles, particularly those prepared in a limited coalescence-type process. For example, such charge agents often fail to provide sufficient initial charge or to maintain such charge for adequate development. A lack of satisfactory charge-control can significantly and detrimentally affect the life of an electrostatographic developer.
It would therefore, be desirable to provide a limited coalescence process for preparing binder polymer particles exhibiting adequate electrostatographic charge-control characteristics. Likewise, it would be desirable to provide electrostatographic toners comprising such binder polymer particles. The present invention meets such objectives.